The red weed (also referred to as the red creeper or the red swamp) is a fictional plant native to Mars in the novel The War of the Worlds by H. G. Wells. It is this plant that supposedly gives Mars its dull red colour. It is one of the several types of plants brought to Earth possibly accidentally by the invading Martians, but the only one that truly was able to adapt and grow widespread on Earth. When it is exposed to water, it grows and reproduces explosively, flooding the neighboring countryside as it clogs streams and rivers. The narrator mentions near the end of "The Man on Putney Hill" that the weed glows purple at night. He tries eating some, but it has a metallic taste. Though it engulfed the native plant life of Earth it also succumbed to the effects of Earth bacteria.
As the book has been interpreted as criticism of imperialism, the red weed could symbolize the non-native fauna colonizers introduced to the Americas, Australia, New Zealand and other countries. In many cases, these introduced species overwhelmed the native fauna, especially in remote islands.
In other adaptations
The red weed is not mentioned at all in the radio adaptation, and is absent from the 1953 film; however its absence fits in with the retcon established for the TV series follow-up in which the aliens originate from Mor-Tax, a garden planet. Therefore, their means of transforming the planet was actually to conserve and promote Earth's own vegetation.
In Jeff Wayne's Musical Version of The War of the Worlds, the red weed is given a two-part song, with eerie music to signify its slow and threatening growth over the Earth. In the live tour, CGI footage of the red weed portrayed it as a mossy creeper that grew much faster than Wells' original account.
In Steven Spielberg's 2005 War of the Worlds film, the presence of the red weed on Earth is clearly intentional. Once they have a strong hold of the planet, the invaders take captured humans and drain their bodily fluids, which act as a fertilizer for the red weed, helping it grow and cover the planet. Spielberg did state that these invaders did not come from Mars (the "Red Planet"), the script indicates the weed is abundantly present on their world as well, suggesting the two share a superficial resemblance. However, what exactly makes up their red weed, whether it is a natural vegetation or what gives it its color on their world, is unknown; though it is believed by a character in the movie the aliens fertilise the weed with human blood, possibly adding to its redness. In one scene, the Tripods used their tentacles to dig fiercely at the ground, which could have been when and how they planted the red weed. It grows so quickly it can be seen growing, and it can grow on just about anything. It is also stated in the script the weed is fierce enough to flourish in spite of the conditions that forced the aliens to find refuge on a new world. But despite its tenacity to survive such a harsh environment, much like in the novel, the red weed is killed by Earth's bacteria and turned into an ash-like dust and is blown away in the wind before it can be examined.
The red weed is also depicted in the Timothy Hines film H.G. Wells' The War of the Worlds, which is a direct adaptation of the novel. However, it is not featured in any detail, and its presence is nothing more than as a part of the background. In fact, the term "red weed" is said only once throughout the film.
The weed doesn't appear in the modernised, 2005 Asylum film H.G. Wells' War of the Worlds (also entitled Invasion), but later scenes in the film show a reddish-orange colour scheme, possibly used to adapt the red weed into a more realistic aspect and to resemble the colour of Mars to symbolise the aliens' control over mankind. It is confirmed that no such weed exists by Mars Rover recordings during the opening title sequence, and the surface of the planet seen on the sequel War of the Worlds 2: The Next Wave.
It is also present in the PC game of The War of the Worlds, where it replaces the trees if more Martian buildings and defenses are built in that sector, as well as the mobile game of Jeff Wayne's Musical Version of The War of The Worlds: Minigame Adventure. In 2012, it received its own Android game called Red Weed, where players try to stop the slimy Red Weed from spreading over the land as it changes every patch of green grass it touches into red tendrils.
In Alan Moore and Kevin O'Neill's comic book series The League of Extraordinary Gentlemen, Volume II, which deals with the effects of the Martian invasion within a wider fictional context, the red weed is deliberately employed by the Martians as an anti-shipping weapon, in order to prevent Captain Nemo's submarine, the Nautilus, from providing effective resistance to the Martian invasion, after the Invisible Man warns them of the Nautilus.
In the comic sequel to the story, Scarlet Traces, the red weed has been farmed because its oil is the only thing that can lubricate the adapted Martian technology. As one character points out, this suggests that the Martians brought it to Earth on purpose.